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language for range clause
(I have deliberately left away the forms w/ := or = and the forms with :) R=r DELTA=106 (44 added, 13 deleted, 49 changed) OCL=21192 CL=21283
This commit is contained in:
Robert Griesemer
parent
ef77c22626
commit
30a1a8c922
141
doc/go_spec.txt
141
doc/go_spec.txt
@ -28,7 +28,6 @@ Timeline (9/5/08):
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Missing:
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[ ] partial export of structs, methods
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[ ] range statement: to be defined more reasonably
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[ ] packages of multiple files
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[ ] Helper syntax for composite types: allow names/indices for maps/arrays,
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remove need for type in elements of composites
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@ -56,7 +55,6 @@ Open issues:
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[ ] old-style export decls (still needed, but ideally should go away)
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[ ] like to have assert() in the language, w/ option to disable code gen for it
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[ ] composite types should uniformly create an instance instead of a pointer
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[ ] semantics of statements
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[ ] need for type switch? (or use type guard with ok in tuple assignment?)
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[ ] do we need anything on package vs file names?
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[ ] type switch or some form of type test needed
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@ -93,6 +91,8 @@ Decisions in need of integration into the doc:
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Closed:
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[x] semantics of statements - we just need to fill in the language, the semantics is mostly clear
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[x] range statement: to be defined more reasonably
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[x] need to be specific on (unsigned) integer operations: one must be able
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to rely on wrap-around on overflow
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[x] global var decls: "var a, b, c int = 0, 0, 0" is ok, but "var a, b, c = 0, 0, 0" is not
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@ -198,7 +198,6 @@ Contents
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If statements
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Switch statements
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For statements
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Range statements
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Go statements
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Select statements
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Return statements
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@ -583,7 +582,9 @@ same identifier declared in an outer block.
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3. The scope of a constant or variable extends textually from
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after the declaration to the end of the innermost surrounding
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block.
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block. If the variable is declared in the init statement of an
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if, for, or switch statement, the innermost surrounding block
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is the block associated with the respective statement.
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4. The scope of a parameter or result identifier is the body of the
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corresponding function.
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@ -2369,8 +2370,7 @@ Statements control execution.
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Statement =
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Declaration | LabelDecl | EmptyStat |
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SimpleStat | GoStat | ReturnStat | BreakStat | ContinueStat | GotoStat |
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FallthroughStat | Block | IfStat | SwitchStat | SelectStat | ForStat |
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RangeStat .
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FallthroughStat | Block | IfStat | SwitchStat | SelectStat | ForStat .
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SimpleStat =
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ExpressionStat | IncDecStat | Assignment | SimpleVarDecl .
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@ -2589,68 +2589,99 @@ If the expression is omitted, it is equivalent to "true".
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For statements
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----
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For statements are a combination of the "for" and "while" loops of C.
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A for statement specifies repeated execution of a block. The iteration is
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controlled by a condition, a for clause, or a range clause.
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ForStat = "for" [ Condition | ForClause ] Block .
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ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] .
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InitStat = SimpleStat .
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ForStat = "for" [ Condition | ForClause | RangeClause ] Block .
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Condition = Expression .
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PostStat = SimpleStat .
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A SimpleStat is a simple statement such as an assignment, a SimpleVarDecl,
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or an increment or decrement statement. Therefore one may declare a loop
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variable in the init statement.
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for i := 0; i < 10; i++ {
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print(i, "\n")
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}
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A for statement with just a condition executes until the condition becomes
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false. Thus it is the same as C's while statement.
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In its simplest form, a for statement specifies the repeated execution of
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a block as long as a condition evaluates to true. The condition is evaluated
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before each iteration. The type of the condition expression must be boolean.
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If the condition is absent, it is equivalent to "true".
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for a < b {
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a *= 2
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}
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If the condition is absent, it is equivalent to "true".
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A for statement with a for clause is also controlled by its condition, but
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additionally it may specify an init and post statement, such as an assignment,
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an increment or decrement statement. The init statement may also be a (simple)
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variable declaration; no variables can be declared in the post statement.
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for {
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f()
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ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] .
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InitStat = SimpleStat .
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PostStat = SimpleStat .
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For instance, one may declare an iteration variable in the init statement:
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for i := 0; i < 10; i++ {
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f(i)
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}
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If present, the init statement is executed once before commencing the iteration;
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the post statement is executed after each execution of the statement block (and
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only if the block was executed). The scope of any variable declared in the init
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statement ends with the end of the for statement block ($Declarations and scope
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rules, Rule 3).
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The init and post statement as well as the condition may be omitted; however
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if either the init or post statement are present, the separating semicolons
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must be present. If the condition is absent, it is equivalent to "true".
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The following statements are equivalent:
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for ; cond ; { S() } is the same as for cond { S() }
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for true { S() } is the same as for { S() }
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Alternatively, a for statement may be controlled by a range clause. A
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range clause specifies iteration through all entries of an array or map.
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For each entry it first assigns the current index or key to an iteration
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variable - or the current (index, element) or (key, value) pair to a pair
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of iteration variables - and then executes the block. Iteration terminates
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when all entries have been processed, or if the for statement is terminated
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early, for instance by a break or return statement.
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RangeClause = IdentifierList ( "=" | ":=" ) "range" Expression .
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The type of the right-hand expression in the range clause must be an array or
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map, or a pointer to an array or map. If it is a pointer, it must not be nil.
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The left-hand identifier list must contain one or two identifiers denoting the
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iteration variables. The first variable is set to the current array index or
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map key, and the second variable, if present, is set to the corresponding
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array element or map value. The types of the array index (int) and element,
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or of the map key and value respectively, must be assignment-compatible to
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the iteration variables.
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The iteration variables may be declared by the range clause (":="), in which
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case their scope ends at the end of the for statement block ($Declarations and
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scope rules, Rule 3). In this case their types are the array index and element,
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or the map key and value types, respectively.
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var a [10]string;
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m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6};
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for i, s := range a {
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// type of i is int
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// type of s is string
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// s == a[i]
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g(i, s)
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}
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Range statements
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----
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Range statements are a special control structure for iterating over
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the contents of arrays and maps.
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RangeStat = "range" IdentifierList ":=" RangeExpression Block .
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RangeExpression = Expression .
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A range expression must evaluate to an array, map or string. The identifier list must contain
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either one or two identifiers. If the range expression is a map, a single identifier is declared
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to range over the keys of the map; two identifiers range over the keys and corresponding
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values. For arrays and strings, the behavior is analogous for integer indices (the keys) and
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array elements (the values).
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a := []int(1, 2, 3);
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m := [string]map int("fo",2, "foo",3, "fooo",4)
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range i := a {
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f(a[i]);
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var key string;
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var val interface {}; // value type of m is assignment-compatible to val
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for key, value = range m {
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h(key, value)
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}
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// key == last map key encountered in iteration
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// val == map[key]
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range i, v := a {
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f(v);
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}
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range k, v := m {
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assert(len(k) == v);
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}
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TODO: is this right?
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If map entries that have not yet been processed are deleted during iteration,
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they will not be processed. If map entries are inserted during iteration, the
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behavior is implementation-dependent. Likewise, if the range expression is a
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pointer variable, the behavior of assigning to that variable is implementation-
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dependent. Assigning to the iteration variables during iteration simply changes
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the values of those variables for the current iteration; it does not affect any
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subsequent iterations.
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Go statements
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